The present invention relates generally to a lightweight hollow metal stainless steel screw design particularly for use in aerospace applications, and a related method of making the hollow metal screw. The hollow metal screw is produced by stamping from a generally circular sheet metal disk in one or more steps to form a hollow tubular head and screw shank having a wall of thickness of from 0.2 to 0.7 millimeters. The screw shank is thereafter annealed to soften the screw shank for thread rolling, after which the hollow metal screw is hardened sufficiently to meet aerospace thread strength requirements in a screw which is lighter than a similar size solid screw and therefore contributes to aircraft fuel economy.
Screws are generally known in the art, and tend to comprise a solid core shank upon which is rolled to form a set of screw threads of prescribed pitch and length. In general terms, a high strength corrosion resistant steel (CRES) material is preferred as the screw material since threads of hardened CRES can be repeatedly re-installed into a threaded bore receptacle without thread damage. Although, CRES constitutes a relatively heavy metal material and thus does not contribute to aircraft fuel economy, especially when the typical aircraft includes several thousand such screws. Attempts to use lighter weight metals, such as a solid core aluminum or titanium screw, have met with limitations in the ability for a mechanic to repeatedly install and remove the same screw without damaging the threads due to thread galling.
In the past, threaded screws have comprised solid cores, or a hollow screw backstopped by a core insert of plastic or the like. Unfortunately, the plastic core insert is not without at least some weight addition, whereby the screw still is limited in its contribution to aircraft fuel economy.
The present invention comprises an improved hollow corrosion resistant metal screw wherein the screw is stamped from a generally circular disk of a selected corrosion resistant metal material to provide a shank wall thickness of 0.2 to 0.7 millimeters, followed by annealing to soften the metal material, followed in turn by thread rolling and then by hardening steps to provide a hollow metal screw with a thread strength sufficient to withstand most aerospace applications, and to contribute to aircraft fuel economy by providing a hollow screw with corrosion resistant metal threads wherein the hollow screw is approximately 50% the weight of a solid core screw made from the same material. The present invention fulfills these needs and provides further advantages.